Magnetic powder, method for production thereof, and magnetic recording medium

ABSTRACT

A method for producing a magnetic powder includes performing a reduction treatment on the surface of particles including a hard magnetic material to form core-shell particles each having a shell portion including a soft magnetic material.

TECHNICAL FIELD

The present technology relates to a magnetic powder including core-shellparticles, a method for production thereof, and a magnetic recordingmedium having a magnetic layer including the magnetic powder.

BACKGROUND ART

Some conventionally known coating-type magnetic recording media have amagnetic layer that is formed by applying, onto a nonmagnetic support, amagnetic coating material including a magnetic powder, a binder, and anorganic solvent, and then drying the coating material. Such coating-typemagnetic recording media are widely used as high-density recording mediasuch as backup data cartridges.

In recent years, magnetic powders for use in magnetic layers have beenmade finer for recording media with higher recording density. However,magnetic powders with further reduced particle sizes can be affected byexternal heat in the environment where magnetic tapes are used, so thatthe influence of what is called thermal agitation of magnetization canbe significant to cause a phenomenon in which the recorded magnetizationdisappears. To avoid the influence of the thermal agitation ofmagnetization, it is necessary to increase the magnetic anisotropy orcoercivity of magnetic powders.

However, an increase in coercivity can make it difficult for recordingheads to cause magnetic reversal, in other words, make it difficult torecord information signals. In addition, magnetic powders with furtherreduced particle sizes may have lower saturation magnetization σs,which, together with a decrease in output due to high-density recording,may cause significant degradation of signal-to-noise ratio orcarrier-to-noise ratio (hereinafter referred to as “CNR”).

To solve these problems, it is proposed that a soft magnetic coatingfilm with a high saturation magnetization σs be formed around hardmagnetic particles so that each hard magnetic particle as a core portioncan be exchange-coupled with the soft magnetic coating film as a shellportion, which makes it possible to control the coercivity to a valuesuitable for recording and to increase the saturation magnetization σswhile high thermal stability is maintained (see Patent Documents 1 to3).

CITATION LIST Patent Documents

Patent Document 1: Japanese Patent No. 5416188

Patent Document 2: Japanese Patent Application Laid-Open No. 2012-027978

Patent Document 3: Japanese Patent Application Laid-Open No. 2011-216838

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

It is an object of the present technology to provide a magnetic powderthat has high thermal stability and can provide high output levels andthe ability to record easily, to provide a method for producing such amagnetic powder, and to provide a magnetic recording medium.

Solutions to Problems

To solve the problems, a first aspect of the present technology isdirected to a method for producing a magnetic powder, the methodincluding subjecting hard magnetic particles to a reduction treatment toform core-shell particles each having a shell portion including a softmagnetic material.

A second aspect of the present technology is directed to a magneticpowder including core-shell particles each including a core portionincluding a hard magnetic material and a shell portion including a softmagnetic material, wherein the soft magnetic material is obtainable byreducing the hard magnetic material.

A third aspect of the present technology is directed to a magneticrecording medium including: a nonmagnetic support; and a magnetic layerincluding a magnetic powder, wherein the magnetic powder includescore-shell particles each including a core portion including a hardmagnetic material and a shell portion including a soft magneticmaterial, and the soft magnetic material is obtainable by reducing thehard magnetic material.

Effects of the Invention

As described above, the present technology makes it possible to obtainhigh output levels, high thermal stability, and the ability to recordeasily.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of thestructure of a magnetic powder according to a first embodiment of thepresent technology.

FIG. 2 is a cross-sectional view illustrating an example of thestructure of a magnetic powder according to a second embodiment of thepresent technology.

FIG. 3 is a cross-sectional view illustrating an example of thestructure of a magnetic recording medium according to a third embodimentof the present technology.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present technology will be described in the followingorder.

-   1 First Embodiment-   1.1 Features of magnetic powder-   1.2 Method for producing magnetic powder-   1.3 Advantageous effects-   2 Second Embodiment-   2.1 Features of magnetic powder-   2.2 Method for producing magnetic powder-   2.3 Advantageous effects-   3 Third Embodiment-   3.1 Features of magnetic recording medium-   3.2 Method for producing magnetic recording medium-   3.3 Advantageous effects

1 First Embodiment

[1.1 Features of Magnetic Powder]

The magnetic powder according to a first embodiment of the presenttechnology includes a collection of magnetic nanoparticles having acore-shell structure (hereinafter referred to as “core-shellparticles”). As illustrated in FIG. 1, the core-shell particles eachinclude a core portion 11 and a shell portion 12 provided around thecore portion 11. The core portion 11 is exchange-coupled with the shellportion 12. At the interface between the core and shell portions 11 and12, there may be a continuous or discontinuous change in theircomposition and/or state or other properties. The magnetic powderaccording to the first embodiment is suitable for use in magnetic layersof magnetic recording media.

(Core Portion)

The core portion 11 has hard magnetism. The core portion 11 includes ahard magnetic material, which may be, for example, a cubic iron oxidesuch as ε iron oxide (ε-Fe₂O₃) or Co-containing spinel ferrimagneticpowder. The ε iron oxide preferably includes, as a main phase, anε-Fe₂O₃ crystal (which may include a crystal derived from ε-Fe₂O₃ bypartially substituting the Fe sites with a metal element M). The metalelement M is, for example, one or more selected from the groupconsisting of Al, Ga, and In. In this regard, when the molar ratio of Mto Fe in the iron oxide is expressed as follows: M:Fe=x:(2−x), 0≦×<1should be satisfied.

In the context of the present technology, unless otherwise specified,the term “ε-Fe₂O₃ crystal” is intended to include not only a pureε-Fe₂O₃ crystal where the Fe sites are not substituted with any otherelement but also a crystal that is derived from ε-Fe₂O₃ by partiallysubstituting the Fe sites with a metal element M and has the same spacegroup as that of the pure ε-Fe₂O₃ crystal (namely, has a space group ofPna2₁).

(Shell Portion)

The shell portion 12 covers at least part of the circumference of thecore portion 11. Specifically, the shell portion 12 may partially coverthe circumference of the core portion 11 or may cover the entirecircumference of the core portion 11. The shell portion 12 preferablycovers the entire surface of the core portion 11 so that the exchangecoupling between the core and shell portions 11 and 12 can be enough toimprove the magnetic properties.

The shell portion 12 is a soft magnetic layer with soft magnetism. Theshell portion 12 includes a soft magnetic material, which may be, forexample, α-Fe or an Fe-based soft magnetic material. The shell portion12 can be obtained by reducing the surface of a hard magnetic particleas a precursor of the core-shell particle.

Specifically, the soft magnetic material in the shell portion 12 can beobtained by reducing the hard magnetic material in the core portion 11.For example, when the core portion 11 includes ε iron oxide as a hardmagnetic material, the shell portion 12 includes α-Fe, which is amaterial obtainable by reduction of ε iron oxide. When the core portion11 includes a Co-containing spinel ferrimagnetic material as a hardmagnetic material, the shell portion 12 includes an Fe-based softmagnetic material, which is obtainable by reduction of the Co-containingspinel ferrimagnetic material.

(Average Particle Size of Core-Shell Particles)

The core-shell particles (magnetic powder) preferably have an averageparticle size (radius) R_(core/shell) satisfying 4.9nm≦R_(core/shell)≦15 nm. If R_(core/shell) is less than 4.9 nm(R_(core/shell)<4.9 nm), it may be difficult to suppress aggregation ofthe core-shell particles. On the other hand, if R_(core/shell) is morethan 15 nm (15 nm<R_(core/shell)), magnetic recording media producedwith the magnetic powder may have high noise. In this case, coatingfilms produced with the magnetic powder may also have degraded surfacequality. In this regard, the average particle size R_(core/shell) of thecore-shell particles is substantially equal to the average particle sizeR_(ini) of the hard magnetic particles as precursors of the core-shellparticles. Therefore, the hard magnetic particles as precursors alsopreferably have an average particle size R_(ini) satisfying 4.9 nm≦R_(core/shell)≦15 nm.

The average particle size R_(core/shell) of the core-shell particles(magnetic powder) can be determined as described below. First, themagnetic powder is photographed with a transmission electron microscope(TEM). Subsequently, 500 core-shell particles are randomly selected inthe resulting TEM photograph and each measured for particle size. Inthis case, the particle size (radius) means the value of half of themaximum distance across the core-shell particle (namely, the maximumparticle diameter). Subsequently, the average particle sizeR_(core/shell) of the core-shell particles is calculated as the simpleaverage (arithmetic average) of the measured particle sizes of the 500core-shell particles.

(Ratio of Half Width of Particle Size Distribution to Average ParticleSize)

The percentage ratio D of the half width D_(half) of the particle sizedistribution of the core-shell particles (magnetic powder) to theaverage particle size D_(ave) of the core-shell particles (magneticpowder) is preferably 40% or less. Specifically, the ratio D is definedby the following formula.

Ratio D(%)=(the half width D_(half) (nm) of the particle sizedistribution of the core-shell particles/the average particle sizeD_(ave) (nm) of the core-shell particles)×100

If the particle size distribution is broad with a ratio D of more than40%, the thickness of the shell portion 12 may fail to be controlleduniformly between the core-shell particles, so that variations inmagnetic properties such as coercivity Hc can occur between thecore-shell particles. Note that in this embodiment, the particle sizedistribution of the core-shell particles is substantially equal to thatof the hard magnetic particles as precursors of the core-shellparticles.

The ratio D of the core-shell particles (magnetic powder) can bedetermined as described below. First, the magnetic powder isphotographed with a TEM. Subsequently, 500 core-shell particles arerandomly selected in the resulting TEM photograph and each measured forparticle size (diameter). The particle size distribution of the magneticpowder is determined from the measured particle sizes. In this case, theparticle size means the maximum distance across the particle (namely,the maximum particle diameter). Subsequently, the median diameter (50%diameter or D50) is determined from the determined particle sizedistribution and used as the average particle size D_(ave). The halfwidth D_(half) of the particle size distribution is also determined fromthe determined particle size distribution. Subsequently, the ratio D iscalculated from the determined average particle size D_(ave) and thehalf width D_(half) of the particle size distribution.

(Coercivity)

The core-shell particles (magnetic powder) preferably have a coercivityHc of 2,000 Oe to 6,000 Oe (2,000Oe≦Hc≦6,000 Oe). If the Hc is less than2,000 Oe (Hc<2,000 Oe), magnetic recording media produced with themagnetic powder may have lower output levels during short-wavelengthrecording (high-density recording). On the other hand, an Hc of morethan 6,000 Oe (6,000 Oe<Hc) may make it difficult for recording heads toperform saturated recording and make it impossible to obtain good CNR.

The coercivity Hc of the core-shell particles (magnetic powder) can bedetermined as described below. First, the M-H loop of the magneticpowder is obtained using a vibrating sample magnetometer (VSM).Subsequently, the coercivity Hc is determined from the resulting M-Hloop.

(Saturation Magnetization)

The core-shell particles (magnetic powder) preferably have a saturationmagnetization σs of 10 emu/g to 100 emu/g (10 emu/g≦σs≦100 emu/g). Ifthe σs is less than 10 emu/g (σs<10 emu/g), magnetic recording mediaproduced with the magnetic powder may have lower output levels and failto achieve good CNR even when the packing density or squareness ratio ofthe magnetic powder is improved. On the other hand, if the σs is morethan 100 emu/g (100 emu/g<σs), magnetic recording media produced withthe magnetic powder may cause reproducing head saturation and generatenoise.

The saturation magnetization σs of the core-shell particles (magneticpowder) can be determined as described below. First, the M-H loop of themagnetic powder is obtained using a VSM. Subsequently, the saturationmagnetization σs is determined from the resulting M-H loop.

(Average Thickness of Shell Portion)

The average thickness δ_(soft) of the shell portion 12 preferablysatisfies 0.4 nm≦67 _(soft)≦11 nm. If the δ_(soft) is less than 0.4 nm(δ_(soft)<0.4 nm), it may be difficult to perform a uniform reductiontreatment on the hard magnetic particles in the magnetic powderproducing method described below, so that the core portion 11 may beinsufficiently exchange-coupled with the shell portion 12. On the otherhand, if the δ_(soft) is more than 11 nm (11 nm<δ_(soft)), thecore-shell particles may have reduced coercivity Hc, so that goodelectromagnetic conversion characteristics may fail to be obtained.

The average thickness δ_(soft) of the shell portion 12 can be determinedas described below. First, the magnetic powder is photographed with aTEM. Subsequently, 500 core-shell particles are randomly selected in theresulting TEM photograph and each measured for the thickness of theshell portion 12. Note that when there are variations in the thicknessof the shell portion 12 in a single core-shell particle, the thicknessof the shell portion 12 is defined as the maximum thickness of the shellportion 12 of the single core-shell particle. Subsequently, the averagethickness δ_(soft) of the shell portion 12 is calculated as the simpleaverage (arithmetic average) of the measured thicknesses of the shellportions 12 of the 500 core-shell particles.

[1.2 Method for Producing Magnetic Powder]

The method according to the first embodiment of the present technologyfor producing a magnetic powder includes performing a reductiontreatment directly on hard magnetic particles as precursors ofcore-shell particles, so that core-shell particles are produced, eachincluding a core portion 11 with hard magnetism and a shell portion withsoft magnetism. Hereinafter, a description will be given of an exampleof the method according to the first embodiment of the presenttechnology for producing a magnetic powder.

(Step of Producing Powder of Hard Magnetic Particles)

First, a powder is produced including a collection of hard magneticparticles with high coercivity and high crystal magnetic anisotropy. Thehard magnetic particles include a hard magnetic material similar to thatin the core portion 11 described above. When particles including ε ironoxide are to be used as the hard magnetic particles, for example, themethod described in Japanese Patent No. 5105503 may be used to producethe particles. The method described in the patent document can make theparticle size distribution sharper than that obtained by generalproduction methods using reverse micelle method.

(Coating Step)

Subsequently, a coating layer is optionally formed on the surface of thehard magnetic particles in order to suppress aggregation of the hardmagnetic particles. Alternatively, a coating layer may be formed inadvance on the surface of the hard magnetic particles in the above stepof producing a powder of hard magnetic particles. The coating layerincludes, for example, silica, alumina, calcia, magnesia, or zirconia.

Hereinafter, a specific example of silica coating will be described.First, a mixture solution is prepared by adding polyoxyethylenenonylphenyl ether and an ammonia solution to cyclohexane and mixingthem. Subsequently, the powder of hard magnetic particles obtained inthe previous step is dispersed in cyclohexane, and then the dispersionis added to the prepared mixture solution. Subsequently,tetraethoxysilane is further added to the mixture solution and thenstirred. The resulting powder is washed with methanol and ethanol. Inthis way, a powder of silica layer-coated hard magnetic particles isobtained.

(Reduction Treatment Step)

Subsequently, the surface of the hard magnetic particles is subjected toa reduction treatment, so that core-shell particles are formed, eachincluding a core portion 11 including the hard magnetic material; and ashell portion 12 including a soft magnetic material. In this regard,when the coating layer is formed on the surface of the hard magneticparticles, the reduction treatment is performed on the surface of thehard magnetic particles through the coating layer.

For example, when the hard magnetic particles include ε iron oxide, thesurface of the particles including ε iron oxide is reduced, so thatcore-shell particles are formed, each including a core portion 11including ε iron oxide; and a shell portion 12 including α-Fe.

The reduction treatment method may be any of a gas phase reductionmethod and a liquid phase reduction method. The gas phase reductionmethod may be, for example, a method of subjecting the surface of thehard magnetic particles to a reduction treatment in a hydrogenatmosphere. The liquid phase reduction method may include, for example,immersing the hard magnetic particles in a solvent, then adding areducing agent to the solvent, and stirring them so that the surface ofthe hard magnetic particles is subjected to a reduction treatment in thesolvent.

(Removal Step)

When the coating step is performed to form the coating layer on thesurface of the hard magnetic particles, the coating layer should befinally removed from the surface of the core-shell particles, forexample, by an acid treatment using an acid solution. The magneticpowder including a collection of core-shell particles is obtained asdescribed above.

[1.3 Advantageous Effects]

The magnetic powder according to the first embodiment includes acollection of core-shell particles each including a core portion 11including a hard magnetic material; and a shell portion 12 including asoft magnetic material. When magnetic recording media are produced withthe magnetic powder, the resulting media having a magnetic layerincluding the magnetic powder can have high output levels and highthermal stability and provide the ability to record easily.

The method according the first embodiment for producing a magneticpowder includes subjecting hard magnetic particles to a reductiontreatment, so that core-shell particles are formed, each having a shellportion 12 including a soft magnetic material. This makes it possible toproduce uniform core-shell particles and to uniformly produce exchangeinteraction between the hard magnetic particle as the core portion 11and the soft magnetic material as the shell portion 12. Therefore, theproperties of the soft magnetic material with a high saturationmagnetization σs can be utilized, so that the resulting core-shellparticle as a whole can have a high saturation magnetization σs. Theability to record easily is also enhanced because the coercivity Hc ofthe core portion 11 itself can be kept at a high level for reliablethermal stability while the coercivity Hc of the core-shell particle asa whole can be controlled to a level suitable for recording. Inaddition, the hard magnetic particle as the core portion 11 can be madelarger than that obtainable by conventional methods, which makes it easyto maintain high coercivity Hc and is advantageous for improving thermalstability.

A general method for producing a magnetic powder includes depositing, bya liquid phase method, a precursor of a soft magnetic material on thesurface of hard magnetic particles for core portions and then subjectingthe precursor to a reduction treatment to form a soft magnetic materialas a shell portion on the surface of the hard magnetic particles. Whenusing this general method for producing core-shell particles, there willbe a high probability that the core portion will fail to be covered withthe shell portion so that the exchange coupling between the core andshell portions will be insufficient. There is also a fear that when thereduction treatment is performed, the surface of the core portion may bepartially exposed without being completely covered with the shellportion, so that only the exposed part of the core portion may bereduced, which may lead to degradation of magnetic properties such assaturation magnetization σs and coercivity Hc.

In contrast, the possibility of insufficient exchange coupling betweenthe core and shell portions is low in the method according to the firstembodiment for producing a magnetic powder because the method accordingto the first embodiment includes subjecting the surface of hard magneticparticles to a reduction treatment to form core-shell particles eachhaving a core portion covered with a shell portion. Also in contrast tothe general method for producing a magnetic powder, the method accordingto the first embodiment is prevented from reducing only an exposed partof the core portion. This means that there is no fear of degradation ofmagnetic properties such as saturation magnetization σs and coercivityHc.

When using the above general method for producing a magnetic powder(core-shell particles), there will be a high probability that the hardmagnetic particles for forming core portions will aggregate during thedeposition of the precursor by the liquid phase method. In this case,the exchange coupling effect may be insufficient, and the magneticvolume may increase so that noise may increase.

In contrast, when including the coating step, the method according tothe first embodiment for producing a magnetic powder (core-shellparticles) can suppress aggregation of the hard magnetic particles asprecursors. This will reduce the probability of insufficient exchangecoupling, the probability of an increase in magnetic volume, and theprobability of an increase in noise.

When using the above general method for producing a magnetic powder, thesize R_(core/shell) of the resulting core-shell particles will benecessarily larger than the size R_(ini) of the hard magnetic particlesprepared for core portions (R_(ini)<R_(core/shell)) because theprecursor of the soft magnetic material is deposited later on thesurface of the hard magnetic particles prepared for core portions.Therefore, to secure good C/N, it is desired to further reduce the sizeof the hard magnetic particles for forming core portions. However, theparticle size reduction may degrade the magnetic properties such ascoercivity Hc and saturation magnetization σs, which are basicallyrequired of hard magnetic particles for use in high-density recordingmedia. In addition, the particle size distribution may vary, or theparticles may aggregate significantly.

In contrast, when using the method according to the first embodiment forproducing a magnetic powder, the size of the core-shell particlesobtained after the reduction treatment will be substantially equal tothe size of the hard magnetic particles used as the precursors(R_(ini)=R_(core/shell)) because the surface of the hard magneticparticles prepared previously are directly subjected to the reductiontreatment in the method according to the first embodiment. Therefore,when the size of the hard magnetic particles as the precursors iscontrolled, core-shell particles with the desired size can besuccessfully produced. Therefore, the method according to the firstembodiment is less likely to cause degradation of magnetic properties,which can be a concern in the above general method for producing amagnetic powder. In addition, the method according to the firstembodiment is less likely to cause variations in particle sizedistribution or aggregation of particles.

2 Second Embodiment

[2.1 Features of Magnetic Powder]

As illustrated in FIG. 2, the magnetic powder according to a secondembodiment of the present technology includes a collection of core-shellparticles each including a core portion 11 and a shell portion 13provided around the core portion 11 and having a two-layer structure.Components similar to those in the first embodiment are denoted by thesame reference signs, and a repeated description thereof will beomitted.

(Shell Portion)

The shell portion 13 with a two-layer structure includes a soft magneticlayer 13 a provided on the core portion 11 and an oxide film 13 bprovided on the soft magnetic layer 13 a.

(Soft Magnetic Layer)

The soft magnetic layer 13 a is similar to the shell portion 12 in thefirst embodiment.

(Oxide Film)

The oxide film 13 b, which is an anti-oxidation layer, includes amaterial that can be obtained by oxidizing the soft magnetic material inthe soft magnetic layer 13 a. Specifically, the oxide film 13 b can beobtained by oxidizing the surface of the soft magnetic layer as aprecursor. When the soft magnetic layer 13 a includes α-Fe as a softmagnetic material, the oxide film 13 b includes at least one of Fe₃O₄,Fe₂O₃, and FeO, which can be obtained by oxidizing α-Fe.

(Average Thickness of Oxide Film)

The oxide film 13 b preferably has an average thickness δ_(barrier)satisfying 0.4 nm≦δ_(barrier)≦11 nm. If the δ_(barrier) is less than 0.4nm (δ_(barrier)<0.4 nm), it may be difficult to control the thickness ofthe oxide film 13 b and also difficult to allow the oxide film 13 b tofunction effectively. On the other hand, if the δ_(barrier) is more than11 nm (11 nm<δ_(barrier)), the shell portion 12 may be relatively thinso that the exchange coupling between the core and shell portions 11 and12 may be insufficient.

The average thickness δ_(barrier) of the oxide film 13 b can bedetermined as described below. First, the magnetic powder isphotographed with a TEM. Subsequently, 500 core-shell particles arerandomly selected in the resulting TEM photograph and each measured forthe thickness of the oxide film 13 b of each core-shell particle. Notethat when there are variations in the thickness of the oxide film 13 bof a single core-shell particle, the thickness of the oxide film 13 b isdefined as the maximum thickness of the oxide film 13 b of the singlecore-shell particle. Subsequently, the average thickness δ_(barrier) ofthe oxide film 13 b is calculated as the simple average (arithmeticaverage) of the measured thicknesses of the oxide films 13 b of the 500core-shell particles.

(Ratio of the Average Thickness of Soft Magnetic Layer to the AverageThickness of Oxide Film)

The ratio (δ_(soft/δbarrier)) of the average thickness δ_(soft) of thesoft magnetic layer 13 a to the average thickness δ_(barrier) of theoxide film 13 b preferably satisfies 0.1≦δ_(soft)/δ_(barrier)≦10. Ifδ_(soft)/δ_(barrier) is either less than 0.1 or more than 10)δ_(soft)/δ_(barrier)<0.1 and 10<δ_(soft)/δ_(barrier)), it may bedifficult to uniformly maintain the thickness of the soft magnetic layer13 a and the oxide film 13 b on the nanometer order, which may make itimpossible to obtain the desired core-shell particles.

The ratio (δ_(soft)/δ_(barrier)) can be determined as described below.First, the average thickness δ_(barrier) of the oxide film 13 b isdetermined as described above. Subsequently, the average thicknessδ_(soft) of the soft magnetic layer 13 a is determined similarly to theaverage thickness δ_(barrier) of the oxide film 13 b, except that thethickness of the soft magnetic layer 13 a is measured instead of that ofthe oxide film 13 b. Subsequently, the ratio (δ_(soft)/δ_(barrier)) iscalculated from the determined average

[2.2 Method for Producing Magnetic Powder]

The method according to the second embodiment of the present technologyfor producing a magnetic powder differs from the method according to thefirst embodiment for producing a magnetic powder in that it furtherincludes a gradual oxidation step after the reduction treatment step andbefore the removal step. Therefore, hereinafter, only the gradualoxidation step will be described.

(Gradual Oxidation Step)

The reduction treatment step described above is followed by subjectingthe core-shell particles to a gradual oxidation treatment under anatmosphere containing a small amount of oxygen for a predetermined time(e.g., 10 minutes) in a high-temperature environment without exposingthe core-shell particles to the air. As a result, the oxide film 13 b isformed on the surface of the core-shell particles. Specifically, theshell portion 13 is formed having the soft magnetic layer 13 a and theoxide film 13 b.

[2.3 Advantageous Effects]

In the magnetic powder according to the second embodiment, thecore-shell particles have the oxide film 13 b at their surface and thusresist rusting of their surface and other damage, which would otherwiseoccur when the surface of the core-shell particles is exposed to theair. Therefore, the magnetic powder has properties resistant todegradation.

3 Third Embodiment

A third embodiment shows a magnetic recording medium having a magneticlayer including the magnetic powder according to the first or secondembodiment.

[3.1 Features of Magnetic Recording Medium]

As illustrated in FIG. 3, the magnetic recording medium according to athird embodiment of the present technology includes a nonmagneticsupport 21, a nonmagnetic layer 22 provided on one principal surface ofthe nonmagnetic support 21, and a magnetic layer 23 provided on thenonmagnetic layer 22. If necessary, the magnetic recording medium mayfurther include a back coat layer 24 provided on the other principalsurface of the nonmagnetic support 21. The magnetic recording mediumaccording to the third embodiment is, for example, a vertical magneticrecording medium for use with a giant magneto-resistance (GMR) head as areproducing head for a recording/reproducing system.

(Nonmagnetic Support)

The nonmagnetic support 21 is, for example, a long flexible film.Examples of materials that can be used to form the nonmagnetic support21 include polyesters such as polyethylene terephthalate, polyolefinssuch as polyethylene and polypropylene, cellulose derivatives such ascellulose triacetate, cellulose diacetate, and cellulose butyrate, vinylresins such as polyvinyl chloride and polyvinylidene chloride,polycarbonate, polyimide, polyamide-imide, and other plastics, lightmetals such as aluminum alloys and titanium alloys, and ceramics such asalumina glass. To increase the mechanical strength, a thin filmincluding an oxide of Al or Cu may be formed at least one of theprincipal surfaces of the nonmagnetic support 21 including, for example,a vinyl resin, and such a thin film-coated support may also be used.

(Magnetic Layer)

The magnetic layer 23 includes, for example, a magnetic powder, abinder, and conductive particles. If necessary, the magnetic layer 23may further include an additive such as a lubricant, an abrasive, or anantirust agent.

(Magnetic Powder)

The magnetic powder is according to the first or second embodimentdescribed above. The average particle size R _(core/shell) of thecore-shell particles, the ratio D(=(D_(half)/D_(ave))×100), the averagethickness δ_(sof) of the shell portion 12, the average thicknessδ_(barrier) of the oxide film 13 b, the value range of the ratio(δ_(soft)/δ_(barrier)) and other features are preferably similar tothose in the first and second embodiments. These values can bedetermined as described below. First, a cross-section is cut from themagnetic recording medium. The magnetic powder in the cross-section isphotographed with a TEM. The values can be determined as described forthe first and second embodiments, except that the resulting TEMphotograph is used.

(Binder)

The binder is preferably a resin having a structure formed by subjectinga polyurethane resin, a vinyl chloride resin, or other resins to acrosslinking reaction. However, the binder is not limited to such aresin, and any other resin may be added as appropriate depending on thephysical properties required of the magnetic recording medium. Any resincommonly used for coating type magnetic recording media may be added.

Examples include vinyl chloride, vinyl acetate, vinyl chloride-vinylacetate copolymers, vinyl chloride-vinylidene chloride copolymers, vinylchloride-acrylonitrile copolymers, acrylic ester-acrylonitrilecopolymers, acrylic ester-vinyl chloride-vinylidene chloride copolymers,vinyl chloride-acrylonitrile copolymers, acrylic ester-acrylonitrilecopolymers, acrylic ester-vinylidene chloride copolymers, methacrylicester-vinylidene chloride copolymers, methacrylic ester-vinyl chloridecopolymers, methacrylic ester-ethylene copolymers, polyvinyl fluoride,vinylidene chloride-acrylonitrile copolymers, acrylonitrile-butadienecopolymers, polyamide resins, polyvinyl butyral, cellulose derivatives(cellulose acetate butyrate, cellulose diacetate, cellulose triacetate,cellulose propionate, nitrocellulose), styrene-butadiene copolymers,polyester resins, amino resins, and synthetic rubber.

Thermosetting resins or reactive resins may also be used, examples ofwhich include phenolic resins, epoxy resins, urea resins, melamineresins, alkyd resins, silicone resins, polyamine resins, andurea-formaldehyde resins.

To improve the dispersibility of the magnetic powder, a polar functionalgroup such as —SO₃M, —OSO₃M, —COOM, or P=O(OM)₂ may also be introducedinto each of the above binders. In the formulae, M is a hydrogen atom oran alkali metal such as lithium, potassium, or sodium.

The polar functional group may also be of a side chain type having an—NR1R2 or —NR1R2R3+X— terminal group or of a main chain typeof >NR1R2+X—. In the formulae, R1, R2, and R3 are each a hydrogen atomor a hydrocarbon group, and X— is a halogen ion such as a fluorine,chlorine, bromine, or iodine ion or an inorganic or organic ion. Thepolar functional group may also be —OH, —SH, —CN, or an epoxy group.

(Conductive particles)

The conductive particles may be fine particles including carbon as amain component, such as carbon black. The carbon black may be, forexample, Asahi #15 or #15HS from Asahi Carbon Co., Ltd. Hybrid carbonparticles composed of silica particles and carbon deposited on thesurface thereof may also be used.

(Nonmagnetic Reinforcing Particles)

The magnetic layer 23 may further contain nonmagnetic reinforcingparticles such as particles of aluminum oxide (α, β, γ), chromium oxide,silicon oxide, diamond, garnet, emery, boron nitride, titanium carbide,silicon carbide, titanium carbide, or titanium oxide (rutile, anatase).

(Nonmagnetic Layer)

The nonmagnetic layer 22 includes a nonmagnetic powder and a binder asmain components. If necessary, the nonmagnetic layer 22 may furtherinclude various additives such as conductive particles and a lubricant.

(Nonmagnetic Powder)

The nonmagnetic powder may include appropriate fine particles of any ofvarious shapes such as needle, spherical, and plate shapes.

(Binder)

The binder may be any of those suitable for use in the magnetic layer 23described above. A combination of a resin and a polyisocyanate may alsobe used to form the nonmagnetic layer 22, in which the resin may becrosslinked and cured with the polyisocyanate. Examples of thepolyisocyanate include toluene diisocyanate and adducts thereof, andalkylene diisocyanate and adducts thereof.

(Conductive Particles)

Like the conductive particles for the magnetic layer 23 described above,the conductive particles for the nonmagnetic layer 22 may include, forexample, carbon black or hybrid carbon particles composed of silicaparticles and carbon deposited on the surface thereof.

(Lubricant)

The magnetic layer 23 and the nonmagnetic layer 22 may contain anyappropriate lubricant, examples of which include esters of a monobasicfatty acid of 10 to 24 carbon atoms with any of monohydric to hexahydricalcohols of 2 to 12 carbon atoms, any mixture of these esters, or di- ortri-fatty acid esters. Specific examples of the lubricant include lauricacid, myristic acid, palmitic acid, stearic acid, behenic acid, oleicacid, linoleic acid, linolenic acid, elaidic acid, butyl stearate,pentyl stearate, heptyl stearate, octyl stearate, isooctyl stearate, andoctyl myristate.

[3.2 Method for Producing Magnetic Recording Medium]

Hereinafter, a description will be given of an example of a method forproducing the magnetic recording medium having the features describedabove.

First, a nonmagnetic layer-forming coating material is prepared bykneading and dispersing the nonmagnetic powder, the conductiveparticles, the binder, and other materials in a solvent. Subsequently, amagnetic layer-forming coating material is prepared by kneading anddispersing the magnetic powder, the conductive particles, the binder,and other materials in a solvent. Similar solvents, dispersing machines,and kneading machines may be used in the preparation of the magneticlayer-forming coating material and the nonmagnetic layer-forming coatingmaterial.

Examples of solvents that can be used for the preparation of the coatingmaterials include ketone solvents such as acetone, methyl ethyl ketone,methyl isobutyl ketone, and cyclohexanone; alcohol solvents such asmethanol, ethanol, and propanol; ester solvents such as methyl acetate,ethyl acetate, butyl acetate, propyl acetate, ethyl lactate, andethylene glycol acetate; ether solvents such as diethylene glycoldimethyl ether, 2-ethoxyethanol, tetrahydrofuran, and dioxane; aromatichydrocarbon solvents such as benzene, toluene, and xylene; andhalogenated hydrocarbon solvents such as methylene chloride, ethylenechloride, carbon tetrachloride, chloroform, and chlorobenzene. Thesesolvents may be used alone or in any mixture.

The coating materials may be prepared using a kneading machine, examplesof which include, but are not limited to, biaxial continuous kneaders,biaxial continuous kneaders capable of multistage dilution, kneaders,pressure kneaders, and roll kneaders. The coating materials may also beprepared using a dispersing machine, examples of which include, but arenot limited to, roll mills, ball mills, horizontal sand mills, verticalsand mills, spike mills, pin mills, tower mills, DCP mills,homogenizers, and ultrasonic dispersers.

Subsequently, the nonmagnetic layer-forming coating material is appliedto one principal surface of the nonmagnetic support 21 and then dried toform the nonmagnetic layer 22. Subsequently, the magnetic layer-formingcoating material is applied onto the nonmagnetic layer 22 and then driedto form the magnetic layer 23 on the nonmagnetic layer 22. Subsequently,a back coat layer-forming coating material is applied to the otherprincipal surface of the nonmagnetic support 21 and then dried to formthe back coat layer 24.

Subsequently, the nonmagnetic support 21 with the nonmagnetic layer 22,the magnetic layer 23, and the back coat layer 24 formed thereon isre-wound on a large diameter core and then subjected to a curingtreatment. Subsequently, the nonmagnetic support 21 with the nonmagneticlayer 22, the magnetic layer 23, and the back coat layer 24 formedthereon is calendered and then cut into pieces with a predeterminedwidth. In this way, pancakes are obtained, which correspond to the cutpieces with the predetermined width. Note that the step of forming theback coat layer 24 may be performed after the calendering.

The steps of forming the nonmagnetic layer 22 and the magnetic layer 23are not limited to the examples described above. For example, thenonmagnetic layer 22 and the magnetic layer 23 may be formed on oneprincipal surface of the nonmagnetic support 21 by a process thatincludes applying the nonmagnetic layer-forming coating material to oneprincipal surface of the nonmagnetic support 21 to form a wet coatingfilm, applying the magnetic layer-forming coating material onto the wetcoating film to form a coating film, and then drying both coating films.

[3.3 Advantageous Effects]

In the magnetic recording medium according to the third embodiment, themagnetic layer 23 includes a collection of the core-shell particles eachincluding the core portion including a hard magnetic material and theshell portion including a soft magnetic material. This allows themagnetic recording medium to have high output levels, high thermalstability, and a good ability to record easily.

The present technology has been described specifically with reference toembodiments. It will be understood that the above embodiments are notintended to limit the present technology and that various modificationsthereof may be made on the basis of the technical idea of the presenttechnology.

For example, the structures, methods, steps, shapes, materials,numerical values, and other features described in connection with theabove embodiments are by way of example only, and may be changed ormodified as needed.

In addition, any combination of the structures, methods, steps, shapes,materials, numerical values, and other features described in connectionwith the above embodiments is possible without departing from the gistof the present technology.

The present technology may also have the following features.

(1)

A method for producing a magnetic powder, the method includingsubjecting hard magnetic particles to a reduction treatment to formcore-shell particles each having a shell portion including a softmagnetic material.

(2)

The method according to item (1) for producing a magnetic powder,wherein the hard magnetic particles include an ε-Fe₂O₃ crystal (whichmay include a crystal derived from ε-Fe₂O₃ by partially substituting theFe sites with a metal element M).

(3)

The method according to item (1) or (2) for producing a magnetic powder,wherein the soft magnetic material is α-Fe.

(4)

The method according to any one of items (1) to (3) for producing amagnetic powder, the method further including

forming a coating layer on the hard magnetic particles before thereduction treatment and

removing the coating layer from the core-shell particles after thereduction treatment.

(5)

The method according to item (4) for producing a magnetic powder,wherein the coating layer includes silica.

(6)

The method according to any one of items (1) to (5) for producing amagnetic powder, wherein the core-shell particles have an averageparticle size R_(core/shell) satisfying the relation 4.9 nmR_(core/shell)≦15 nm, and the shell portion has an average thicknessδ_(soft) satisfying the relation 0.4 nm≦δ_(soft)11 nm.

(7)

The method according to any one of items (1) to (6) for producing amagnetic powder, the method further including subjecting the core-shellparticles to a gradual oxidation treatment after the reductiontreatment.

(8)

The method according to item (7) for producing a magnetic powder,wherein

the shell portion after the gradual oxidation treatment includes a softmagnetic layer and an oxide film, and

the oxide film and the soft magnetic layer have an average thicknessδ_(barrier) and an average thickness δ_(soft), respectively, satisfyingthe relations 0.4 nm≦δ_(barrier)11 nm and 0.1≦δ_(soft)/δ_(barrier)≦10,wherein δ_(soft)/δ_(barrier) is the ratio of the average thicknessδ_(soft) of the soft magnetic layer to the average thickness δ_(barrier)of the oxide film.

(9)

The method according to any one of items (1) to (8) for producing amagnetic powder, wherein the core-shell particles have a percentageratio D of D_(half) to D_(ave) of 40% or less, whereinD=(D_(half)/D_(ave))×100, D_(ave) is the average particle size of thecore-shell particles, and D_(half) is the half width of the particlesize distribution of the core-shell particles.

(10)

The method according to items (1) to (9) for producing a magneticpowder, wherein the core-shell particles have a coercivity Hc satisfyingthe relation 2,000 Oe≦Hc≦6,000 Oe.

(11)

The method according to any one of items (1) to (10) for producing amagnetic powder, wherein the core-shell particles have a saturationmagnetization σs satisfying the relation 10 emu/g≦σs≦100 emu/g.

(12)

A magnetic powder including a product obtained by the method accordingto any one of items (1) to (11).

(13)

A magnetic recording medium including:

a nonmagnetic support; and

a magnetic layer including a magnetic powder,

wherein the magnetic powder is according to item (12).

(14)

A magnetic powder including core-shell particles each including

a core portion including a hard magnetic material and

a shell portion including a soft magnetic material,

wherein the core-shell particles are obtainable by reducing particlesincluding a hard magnetic material.

(15)

A magnetic recording medium including:

a nonmagnetic support; and

a magnetic layer including a magnetic powder,

wherein the magnetic powder includes core-shell particles each includinga core portion including a hard magnetic material and a shell portionincluding a soft magnetic material, and the core-shell particles areobtainable by reducing particles including a hard magnetic material.

REFERENCE SIGNS LIST

-   11 Core portion-   12, 13 Shell portion-   13 a Soft magnetic layer-   13 b Oxide film-   21 Nonmagnetic support-   22 Nonmagnetic layer-   23 Magnetic layer-   24 Back coat layer

1. A method for producing a magnetic powder, the method comprisingsubjecting hard magnetic particles to a reduction treatment to formcore-shell particles each having a shell portion comprising a softmagnetic material.
 2. The method according to claim 1 for producing amagnetic powder, wherein the hard magnetic particles comprise an ε-Fe₂O₃crystal (which may include a crystal derived from ε-Fe₂O₃ by partiallysubstituting Fe sites with a metal element M).
 3. The method accordingto claim 2 for producing a magnetic powder, wherein the soft magneticmaterial is α-Fe.
 4. The method according to claim 1 for producing amagnetic powder, the method further comprising forming a coating layeron the hard magnetic particles before the reduction treatment andremoving the coating layer from the core-shell particles after thereduction treatment.
 5. The method according to claim 4 for producing amagnetic powder, wherein the coating layer comprises silica.
 6. Themethod according to claim 1 for producing a magnetic powder, wherein thecore-shell particles have an average particle size R_(core/shell)satisfying the relation 4.9 nm≦R_(core/shell)≦15 nm, and the shellportion has an average thickness δ_(soft) satisfying the relation 0.4nm≦δ_(soft)≦11 nm.
 7. The method according to claim 1 for producing amagnetic powder, the method further comprising subjecting the core-shellparticles to a gradual oxidation treatment after the reductiontreatment.
 8. The method according to claim 7 for producing a magneticpowder, wherein the shell portion after the gradual oxidation treatmentcomprises a soft magnetic layer and an oxide film, and the oxide filmand the soft magnetic layer have an average thickness δ_(barrier) and anaverage thickness δ_(soft,) respectively, satisfying the relations 0.4nm≦δ_(barrier)≦11 nm and 0.1≦δ_(soft)/δ_(barrier)≦10, whereinδ_(soft)/δ_(barrier) is the ratio of the average thickness δ_(soft) ofthe soft magnetic layer to the average thickness δ_(barrier) of theoxide film.
 9. The method according to claim 1 for producing a magneticpowder, wherein the core-shell particles have a percentage ratio D OfD_(half) to D_(ave) of at most 40%, wherein D=(D_(half)/D_(ave))×100,D_(ave) is the average particle size of the core-shell particles, andD_(half) is the half width of the particle size distribution of thecore-shell particles.
 10. The method according to claim 1 for producinga magnetic powder, wherein the core-shell particles have a coercivity Hcsatisfying the relation 2,000 Oe≦Hc≦6,000 Oe.
 11. The method accordingto claim 1 for producing a magnetic powder, wherein the core-shellparticles have a saturation magnetization σs satisfying the relation 10emu/g≦σs≦100 emu/g.
 12. A magnetic powder comprising core-shellparticles each comprising a core portion comprising a hard magneticmaterial and a shell portion comprising a soft magnetic material,wherein the soft magnetic material is obtainable by reducing the hardmagnetic material.
 13. A magnetic recording medium comprising: anonmagnetic support; and a magnetic layer comprising a magnetic powder,wherein the magnetic powder comprises core-shell particles eachcomprising a core portion comprising a hard magnetic material and ashell portion comprising a soft magnetic material, and the soft magneticmaterial is obtainable by reducing the hard magnetic material.